Cladding-pumped optical waveguide
The invention relates to a high power amplifier waveguide for amplifying an optical signal wherein photo darkening due to high optical flux is reduced considerably. This is achieved by providing a cladding pumped amplifier waveguide wherein the optical mode overlap to the active material of the waveguide is low and/or wherein the active material is distributed over a large cross sectional region of the waveguide.
1. A high power amplifier waveguide system for amplifying an optical signal comprising:
a) a diode bar array pump laser which operates at a wavelength λ pump and with a pump power exceeding 100 W;
b) a coupling device;
c) a cladding pumped amplifier waveguide;
d) an output delivery fibre;
the cladding pumped amplifier waveguide comprising
a signal core adapted to guide an optical signal at a signal wavelength, λ signal, the signal core having an effective refractive index n core-1 , wherein the signal core comprises at least one photosensitive region comprising at least one photosensitive element, wherein the photosensitive element is selected from the group of Germanium (Ge), Boron (B), or combinations thereof;
an inner cladding region for guiding pump light, the inner cladding region comprising a first inner cladding surrounding the signal core and having an effective refractive index n inner-clad-1 , the first inner cladding comprising a material doped with at least one rare earth element, the rare earth element being Ytterbium (Yb) and the first inner cladding material is down-doped with an index lowering element selected from the group of Fluorine (F), Boron (B), or combinations thereof, wherein said first inner cladding comprises at least one rare earth doped silica region;
where the wavelength λ pump is resonant with an absorption band of the rare earth element, where n core-1 >n inner-clad-1 , and where an area of the signal core is larger than 50 μm 2 , wherein the signal wavelength, λ signal, of the optical signal is within the interval from 1050 nm to 1090 nm; and
wherein an overlap of the optical signal to the at least one rare earth doped silica region is less than or equal to 25%.
2. The system according to claim 1 , wherein the high power amplifier waveguide system is a waveguide laser system.
3. The system according to claim 2 , wherein between the coupling device and the cladding pumped amplifier waveguide is formed a first fibre Bragg grating, and wherein between the cladding pumped amplifier waveguide and the output delivery fibre is formed a second fibre Bragg grating.
4. The system according to claim 3 , wherein the Bragg gratings are written directly into the amplifier waveguide.
5. The system according to claim 1 , the waveguide further comprising an outer cladding region surrounding the inner cladding region, the outer cladding region comprising at least a first outer cladding having an effective refractive index n outer-clad-1 .
6. The system according to claim 1 , wherein at least part of the rare earth doped silica regions are surrounded by down-doped silica regions, and wherein n inner-clad-1 is given by the average refractive index of the rare earth doped silica regions and the down-doped silica regions.
7. The system according to claim 1 , wherein the inner cladding region further includes a second inner cladding having an effective refractive index n inner-cladding-2 and wherein n inner-clad-1 −n inner-clad-2 is less than 1·10 −3 .
8. The system according to claim 1 , wherein the effective mode area of the optical signal is larger than 50 μm 2 .
9. The system according to claim 1 , where an overlap of the optical signal to the rare earth doped silica region(s) is 25%.
10. The system according to claim 1 , wherein the area of the first inner cladding is larger than about 100 μm 2 .
11. The system according to claim 1 , wherein the ratio between the area of the first inner cladding and the area of the signal core is in the range of about 4 to about 500.
12. The system according to claim 1 , wherein the mode field intensity of the optical signal in the inner cladding region is in the range of about 0.5% to 5%.
13. The system according to claim 1 , wherein the inner cladding features comprise regions with refractive index below that of the first and/or second inner cladding background material, such as voids.
14. The system according to claim 5 , wherein the outer cladding region comprises a polymer coating.
15. A method for fabricating a cladding pumped amplifier waveguide for use in the high power amplifier waveguide system according to claim 1 , comprising arranging precursor elements in a preform and drawing the preform into an amplifier waveguide.